In a conventional liquid crystal display device that drives matrix electrodes using line inversion, a counter electrode driving signal is controlled to be inverted every horizontal period and every vertical period. Further, the counter electrode driving signal may be fixed at “Lo” during a retrace period in order to reduce differences in luminance among gate lines and to improve display quality; and the polarity of the counter electrode driving signal may be adjusted in order to prevent flicker and image burn on the panel, etc.
Here, in the liquid crystal display device driven using line inversion, the counter electrode driving signal is inverted every horizontal period and every vertical period. Thus, where a vertical period consists of n (n is an odd number) horizontal periods, a period in which the counter electrode driving signal is “Hi” (or “Lo”) may be generated longer than the other period during one vertical period.
In this case, a liquid crystal panel serves as a condenser, and thus amplitude of the counter electrode driving signal causes piezoelectricity. This accordingly generates low-frequency vibration, and makes a sound from a panel surface (hereinafter, the sound is referred to as a “low-frequency sound”).
FIG. 7 is a block diagram schematically showing an arrangement of a liquid crystal display device 101 in accordance with a conventional technique.
In the liquid crystal display device 101, a control IC 103 generates a gate driver driving signal GDD, a source driver driving signal SDD, and a counter electrode generating signal REVC in response to a horizontal synchronizing signal HSY, a vertical synchronizing signal VSY, a data clock input signal DCK, and an image data input signal RGB. Then, the gate driver driving signal GDD is supplied to a gate driver 112, and the source driver driving signal SDD is supplied to a source driver 113. Further, a counter electrode signal generating circuit 104 generates a counter electrode driving signal OED in response to the counter electrode generating signal REVC supplied from the control IC 103, and then supplies the counter electrode driving signal OED to a counter electrode (not shown) of a TFT panel 111.
Next, with reference to FIGS. 8 and 9, a general input signal for progressive display will be explained. FIGS. 8 and 9 show the vertical synchronizing signal VSY, the horizontal synchronizing signal HSY, the counter electrode generating signal REVC, and a period for displaying a first line of video data, in the liquid crystal display device 101. The counter electrode generating signal REVC is inverted in synchronism with the horizontal synchronizing signal HSY.
FIG. 8 shows a case where one vertical period consists of an even number of horizontal periods. As shown in FIG. 8, the control IC 103 controls the counter electrode generating signal REVC to be “Hi” when a first vertical synchronizing signal VSY is supplied, and to be “Lo” when a second vertical synchronizing signal VSY is supplied. In this case, the “Hi” period and the “Lo” period of the counter electrode generating signal REVC have the same length during one vertical period. As a result, the low-frequency sound from the surface of the TFT panel 111 is not audible.
FIG. 9 shows a case where one vertical period consists of an odd number of horizontal periods. As shown in FIG. 9, the control IC 103 controls the counter electrode generating signal REVC to be “Hi” when a first vertical synchronizing signal VSY is supplied, and to be “Lo” when a second vertical synchronizing signal VSY is supplied. In this case, however, one of the “Hi” and “Lo” periods of the counter electrode generating signal REVC becomes longer than the other during one vertical period. As a result, the low-frequency sound from the surface of the TFT panel 111 is audible.
Conventionally, to address this low-frequency sound, by inserting a tantalum condenser between the counter electrode driving signal and a ground terminal, the low-frequency sound is converted into an unnoticeable sound, thereby reducing the “sounding” on the panel surface. Further, by covering the liquid crystal display device with a housing to cover the panel surface, the “sounding” is reduced.
Further, Japanese Unexamined Patent Publication No. 133424/1999 (Tokukaihei 11-133424; published on May 21, 1999; hereinafter referred to as “Patent Publication 1”) discloses a liquid crystal display device, which aims to reduce the sounding of an EL light-emitting element used as a backlight, and to reduce the thickness of the liquid crystal display device. In this liquid crystal display device, a daubed copper foil pattern is formed on a surface that faces the EL light-emitting element on a printed substrate, and the copper foil pattern is electrically connected with a front electrode so as to apply a common mode AC voltage.
However, the conventional method to insert the tantalum condenser between the counter electrode driving signal and the ground terminal or to cover the panel surface with the housing has the following problems. For example, this brings about an unnecessary cost, and, since this does not remove the fundamental cause of the low-frequency sound, the low-frequency sound occurs again when an operating frequency is varied in the liquid crystal display device. Further, the Patent Publication 1 does not describe a method to reduce the sounding from a viewpoint of circuitry.